The present invention relates generally to printed circuit boards (PCBs). In particular, the present invention relates to methods of forming printed circuit boards which have optical functionality, by incorporation into the circuit boards optical waveguides formed on a substrate separate from the printed circuit board.
Optical waveguides include a core and a clad surrounding the core, with optical radiation propagating in the core due to its higher index of refraction as compared to the clad. Embedded optical waveguides may be formed on a printed circuit board substrate by coating a first clad layer on the substrate, coating a core layer on the first clad layer, patterning the core layer into one or more core structures, and forming a second clad layer over the first core layer and core structures. The waveguide core layer is typically patterned using standard photolithography and etching processes with a photoresist, or by exposing and developing a photoimageable core layer. For example, U.S. Pat. No. 6,731,857 and U.S. Published Application No. US20040105652, to Shelnut et al, disclose embedded optical waveguides formed with silsesquioxane chemistry on a printed circuit board substrate.
Formation of the optical layers at the same site as the other circuit board processes may have deleterious effects on the resulting optical structures. For example, many of the standard circuit board manufacturing processes, such as plating and drilling processes, generate particulates and other forms of contamination. This contamination may become incorporated into the waveguide layers during formation of the waveguide structures, adversely affecting light transmission properties of the waveguides, for example, by increasing optical loss through light scattering and/or absorption. This is particularly problematic for the waveguide core layer. Such contamination may also give rise to defects during core patterning, as the waveguide structures generally require higher resolution patterning techniques than are typical for standard PCB manufacture. It would therefore be desirable to have methods that allow for the manufacture of waveguides in an environment protected from contamination generated by other circuit board manufacturing processes.
There is thus a need in the optoelectronics industry for improved methods of forming printed circuit boards having optical functionality which overcome or conspicuously ameliorate one or more of the problems described above.